When you share your post or page to social media the Featured Image goes with it and shows up in the listing.
Additional Choice is to Upload an Image Through the Add Media Button
You can also simply upload an image to the text box and click to choose the size as medium, large, or extra large. Do not choose thumbnail as this will cut off part of the image. When you upload an image through the Add Media button in the upper left of the text box you can click to align it on the left, in the center or on the right. It has been my experience that these images also show up on social media when the page is shared.
The link to add the image is located on the right hand side of the Page screen.
WordPress Twenty Twelve Featured Image Size and Steps: There have been a lot of questions from visitors about this. The Featured Image size, like the header height, is flexible. The height and width can be created in the software by pixel size when you create the image. I created my featured image with the collage tool in PIcasa. It is the same way I created the headers. In the set image size area I chose to create a custom size and made it 300 pixels wide and 600 pixels long. This worked with the layout that appears on the left side of the page that includes text and a list of related posts. The bottom widgets came out just below and it all looked even. To create a narrower or shorter image just choose to type in a different smaller pixel size for width and length. Then upload to the site.
When you add a featured image to the Front Page Template it does not show up in the text box the way it would if you added it as an image through the Add Media button to your page or post. But it does show up on the front of your website.
You can also add Featured Images to a Page or Post that is not the Homepage. If the page or post is shared on social media the Featured Image can show up with the shared content.
Steps to Add a Featured Image to a Homepage or Another Page or Post
1. Add New Post or Page. Go to your Dashboard and Click Page or Post depending on which you plan to add. Remember Pages get automatically listed across the top and Posts are the content that show up with the newest content at the top of a list or column of posts. (You can also create a custom menu for the top navigation bar and add the categories for both posts and pages but that is in another tutorial and another video.)
2. Create the Content. Write your Page or Post and upload any photos, graphics or videos that you wish to add to it.
3. Featured Image Upload. Now to go Featured Image in the lower right side of the screen and click to upload a featured image. This image will not pop up showing on the text box but it will show up on the front end of the website. If you have other images already in the post these will be rearranged to make room for the Featured Image.
4. Browse to Computer or Use Media Library. The Featured Image can be added by browsing to get one from your computer or by clicking to get one from the media library which has images you have already uploaded.
5. Select size of Featured Image. There will be size choices for your featured image. When you choose it from the media be sure not to click to add it to the post or page while you are in the media box but instead click on the text for adding a featured image instead.
6. Creating a Featured Image in Graphics Software. Add a photo, graphic art, or a collage that you have created which might also have some text. One of the free easy programs where you can create a custom image is Picasa.
7. Choosing several images for a collage. Click to select some images, click Create in the menu across the top, then select Collage, choose the size and shape from the box that opens, drag and resize images, Create and Save, then double click the image to get the various tools and format it and add text. Upload it as a featured image and there you have created and added a custom image.